In a conventional image forming device, an image forming system is supported to a frame, and an image forming unit which is a part of the image forming system is detachably mounted on the frame. For example, with regard to a tandem type color laser printer having four developing cartridges corresponding to each color of black, yellow, magenta, and cyan, proposal has been made that four sets of developing cartridges and photosensitive drums are detachably mounted on a frame as an integral image forming unit. In this case, improved maintenance efficiency can be obtained because four developing cartridges can be integrally removed from the frame.
Laid open Japanese patent application publication No. 2007-178657 discloses an image forming device in which an image forming unit has a first positioning part and a frame has a second positioning part. Upon engagement of the first positioning part with the second positioning part when inserting the image forming unit into the frame, the image forming unit can be subjected to positioning relative to the frame. More specifically, each shaft protrudes from each lateral side of the image forming unit as the first positioning part, and each hole is formed in each lateral side of the frame as the second positioning part. Positioning of the image forming unit is made by the engagement with each shaft with each hole.
With this structure, however, the shaft directly abuts against the hole when attaching the image forming unit to the frame. Therefore, rough handling of the image forming unit in case of repeated detachment and attachment may cause frictional wearing of the shaft and the hole thereby degrading positioning accuracy.